This application claims the priority of German Application No. 101 01 604.2 filed Jan. 16, 2001, which is incorporated herein by reference.
This invention relates to a metallic cylinder head gasket for an internal-combustion engine. The gasket has at least one sealing layer and a spacer layer and is further provided with openings which, when the gasket is installed, are in alignment with the combustion chambers of the engine. The gasket has a flat portion which surrounds the openings. The sealing layer has a bead having an inner and an outer bead foot and a bead crest (full bead) which, adjoining the flat region, surrounds the respective openings. The spacer layer is provided with an annular overlay whose inner circumferential face is flush with the edge of the respective gasket opening and whose outer circumferential face extends radially up to the region of the bead crest.
The sealing principle of metallic sealing gaskets is based on the presence of an elastic sealing zone. Therefore, about the combustion chambers individual layers of the gasket are provided with beads which form elastic sealing zones. During operation the beads are exposed to continuous pressure changes. The spring force of the spring stiffness of the beads is affected by the shape of the beads and the thickness of the metal layer. The spring characteristic of the beads may be changed only within narrow limits. The region about the combustion chambers is, during operation of the internal-combustion engine, exposed to a higher stress than the remaining surface region of the gasket. To seal such a region securely, it is necessary to apply at that location a high surface pressure which is effected by increasing the material thickness of the gasket along the edge of the combustion chamber. This is achieved, for example, by folding over a metal layer or by providing separate overlays which are bonded to one of the metal layers. At the same time, in many modes of application, the increase of thickness is so designed that in the installed state the beads are protected against a full flattening caused by pressure.
A cylinder head gasket of the above-outlined type is described, for example, in German Offenlegungsschrift (application published without examination) No. 197 51 293. The folded-over metal portion constitutes a pressure limiter for the bead and forms a zone where higher pressure prevails. The bead, however, is situated off the maximum pressure location which is in the area of the thickened material portion and not in the region of the bead. Such a sealing concept has been proven to be satisfactory in diesel engines.
In automotive vehicles operating with Otto engines, however, such a concept is not followed and thus a pressure limiter has been completely abandoned. Thus, the cylinder head gaskets are provided only with beads which surround the combustion chambers. In the installed state of the gasket the beads are completely flattened and take up the maximum pressure, that is, the beads lie in the principal force path.
In European Patent No. 0 574 166, to which corresponds U.S. Pat. No. 5,427,388, a combination of both structural variants is disclosed. The cylinder head gasket disclosed therein has an annular overlay (shim) which in the installed state reaches up to the region of the full bead of the sealing layer. By virtue of such a design, the bead lies in the principal force path. Such a structure is particularly adapted for engines having small web widths between the individual combustion chambers and nevertheless ensures the concentration of a maximum pressure to the region about the combustion chambers.
By virtue of the fact, however, that the annular overlay follows the contour of the bead and does not project beyond the bead crest, the spring characteristic of the bead is changed; the bead becomes harder and thus loses some of its elasticity. Further, in the installed state an extreme contact pressure peak exists in the region of the bead crest. In analogy, such a location represents approximately the support point for a bending beam. Thus, upon pressure application, a roll-off effect over the bead crest occurs, resulting in high oscillation amplitudes in the vertical direction. Such an effect is increased, dependent on the pressures in the combustion chamber.
It is an object of the invention to provide an improved cylinder head gasket having a dynamic sealing zone which lies in the principal force path and which has a high elasticity to ensure that the gasket may be used for heavy-duty diesel engines.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the metal cylinder head gasket includes a spacer plate; a sealing layer superposed on the spacer plate; an opening in the spacer plate and the sealing layer for surrounding a combustion chamber of an internal-combustion engine and a full bead provided in the sealing layer and surrounding the opening. The full bead has a crest, and first and second bead feet situated radially inward and, respectively, radially outward of the crest. An annular overlay surrounds the opening and is positioned between the spacer plate and the sealing layer and is in engagement with the full bead. The annular overlay has a radially inner edge face in alignment with the opening edges; and a radially outer edge face situated between the crest and the second bead foot. The sealing layer further has a half bead surrounding the full bead and being spaced radially outward from the second bead foot.
The measures according to the invention ensure that, on the one hand, the spring characteristics in the installed condition remain unchanged and, on the other hand, the contact pressure peak is relocated into the region of the outer bead foot. The adjoining half bead takes up one part of the contact pressure peak, whereby an overall lower stress is obtained for the full bead.
The combination of the features according to the invention is adapted to significantly improve the static pressure conditions. The vertical sealing gap motion in the region of the full bead is significantly reduced, thus enhancing the service life of the cylinder head gasket.